JB has hit the nail on the head. It's more of an Apple Scroll Bar issue than an Excel issue. Open 'System Preferences' either from your dock or from the Apple Menu (top left), select 'General', then next to the 'Show Scroll Bars' option select 'Always'. When you then open Excel and try and import a text file, the horizontal scroll bar is BACK!! This has frustrated me for 12 months and it now works.

The horizontal scrolling on the Text Import Wizard Step 3 of 3 is also a major issue for me as I use it almost every day. At the moment I have to use an older version of Excel on a different Mac to import .txt files which means it is preventing me from upgrading my other Macs until this is resolved.